Use vk::Result instead of RawResult where applicable

Hi, I was trying to use your library on MacOS and found out that you forgot to port a few usages of RawResult to vk::Result. This caused compile errors due to RawResult not existing (anymore?). I have fixed those in this merge request and verified that your triangle example works on my MBP.

Edit: I did not add a use statement to the file in question as it would result in compile warnings when vk::Result is not used in other configurations.